1. Field of the Invention
This invention relates to an electrical means to measure over a wide range the ratio of the concentrations of oxygen and other oxidizing gaseous species to the concentrations of the various reducing gaseous species such as hydrocarbons, hydrogen and carbon monoxide as might be found in the automotive exhaust.
2. Prior Art
Under many operating conditions, it is desirable to keep the A/F (the ratio of the mass of air to the mass of fuel) at the input to the cylinders of an internal combustion engine near the so called stoichiometric value. At this value harmful exhaust emissions are minimized (and thus most easily reduced further by an exhaust gas catalyst), because there is just enough oxygen present to react with all of the injected hydrocarbons.
To maintain a tight tolerance on A/F control over the life of the car, widespread use has been made of feedback control methods using the so called exhaust gas A/F sensors as the feedback element. These high temperature, solid state devices detect various aspects of the ratio of the concentrations of oxidizing species (mostly oxygen) and reducing species (a mixture of hydrocarbons, hydrogen and carbon monoxide) in the automotive exhaust. Under steady state conditions these aspects are in turn proportional to comparable aspects of the A/F at the input to the cylinders.
Stoichiometric A/F sensors have a step-like transfer function in which a high/low output is obtained when the ambient gas is, for example, rich/lean of stoichiometry. A very sharp transition between the extreme outputs occurs in a narrow A/F region around stoichiometry. Variations in the output in the regions rich or lean of stoichiometry are generally quite small. One consequence of this type of transfer function is that feedback is accomplished in a limit cycle or oscillatory manner. Thus the A/F oscillates with a period on the order of a second about the stoichiometric A/F. These sensors are fabricated from oxide materials and typically employ either an electrochemical or resistive mechanism. For example, the oxygen ion conducting ZrO.sub.2 doped with Y.sub.2 O.sub.3 is the solid electrolyte used in combination with catalytically active Pt electrodes in most electrochemical devices. Porous TiO.sub.2 dosed with fine grains of Pt particles is frequently used for the resistive devices.
In appropriate circumstances it is desirable to operate rich (excess fuel) or lean (excess air) of the stoichiometric A/F. Thus rich conditions may be required for "cold start" and high "load." Lean conditions favor fuel economy. To obtain the advantages of feedback control under various circumstances, it is desirable to have a wide-range A/F sensor. There are known a variety of electrochemical structures made from the ZrO.sub.2 material mentioned above which combine the processes of oxygen pumping and emf measurements using the oxide cells to obtain a wide range of A/F measurements both rich and lean of stoichiometry with a near linear transfer function. See, for example, those described in U.S. Pat. No. 4,272,329 to Hetrick or a publication by Ueno et al. in "Wide-Range Air-Fuel Ratio Sensor" in SAE Paper No. 860409.